Conventionally, a disc brake and/or a drum brake are utilized as the automotive brake device, and a disc brake pad and a brake shoe made by fixing the friction material on a base member made of metal such as steel are utilized as a friction member of the brake device.
The friction material is classified into a semi-metallic friction material containing, as a fiber base, 30 weight % or more but less than 60 weight % of a steel fiber relative to the total amount of the friction material composition, a low steel friction material containing a steel fiber in a part of the fiber base as well as less than 30 weight % of the steel fiber relative to the total amount of the friction material composition, and Non-Asbestos-Organic (NAO) material containing no steel-based fiber such as the steel fiber and a stainless steel fiber.
The friction material generating less braking noise is demanded of late years, it is a recent trend to use the friction material member that uses the NAO friction material that does not contain the steel fiber and steel-based fiber but mainly contains a fiber base such as a nonmetallic fiber, an organic fiber, and an inorganic fiber, a binder such as a thermosetting resin, and a friction modifier such as an organic filler, an inorganic filler, an inorganic abrasive material, a lubricant, and a metallic particle.
Also, because of the environmental consciousness, the friction material which is free of copper, which is one of heavy metals, has been on demand, and the development of the NAO friction material, which contains material which is free of copper component such as a metal copper, a copper alloy, and a copper compound, has been started.
The Patent Document 1 (US Provisional Patent Publication No. 2010/0084233) discloses the friction material for an automotive brake that contains 12-24 volume % of the binder, 2-10 volume % of the fiber, 5 volume % or less of at least one type of lubricant, 15-30 volume % of at least one type of abrasive material, 10-24 volume % of at least one type of titanate and is essentially free of copper and asbestos, i.e., the NAO friction material which is free of copper component.
In the NAO friction material, the copper component, especially a copper fiber and/or a copper particle, have been added as a necessary component to satisfy the required performance, but it became more obvious that removing the copper fiber and copper particle from the NAO friction material causes various problems that did not exist before.
One of the problems is an occurrence of the metal catch. The metal catch occurs when the abrasion dust of the counter member of the friction material that is generated when the friction material frictionally slides on the counter member such as the disc rotor and brake drum made of cast iron, transfer to adhere to the metal component existing on the frictional surface of the friction material to form metal lump, and the metal lump is pushed into and affixed on the internal portion of the frictional surface of the friction material.
When the metal catch occurs, the metal lump inside the friction material significantly grinds the counter member to cause vibration when braking and the life span of the friction material is shortened because of abnormal wear of the friction material due to the grinded counter member.
Conventional friction material that contains a copper fiber and a copper particle, to prevent the metal catch, contain the metal simple substance except copper and alloy that are softer than the counter member and/or the metal sulfide having lubricity.
The Patent Document 2 (Japanese Provisional Patent Publication No. 2002-226834) discloses non-asbestos friction material characterized in manufacturing by forming and curing the non-asbestos friction material composition mainly including a fiber base, filler, and a binder as the frictional material component, and further including tin and/or tin sulfide as the friction material component.
The Patent Document 3 (Japanese Provisional Patent Publication No. 2004-35281) discloses the friction material characterize in manufacturing by forming and curing the non-asbestos friction material composition mainly including fiber base, filler, and binder as the friction material component, and further including 5.5-17.5 volume % of at least three materials that are chosen from tin, tin alloy and tin compound and have different melting points.
However, in the NAO friction material which is free of a copper fiber and a copper particle, it became obvious that the metal catch occurs more when the metal simple substance, alloy, and/or some metal sulfides that have been known to prevent the metal catch are added.